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More than 100 million people will be glued to their TV screens on Sunday, when the Denver Broncos take on the Seattle Seahawks in America's premier sporting contest, the Super Bowl.

But two fierce rivals, Facebook Inc and Twitter Inc , will also be clashing head-to-head on a "second screen" that TV viewers will tune in for the big game.

The two social networks are jostling to be the venue of choice for fans to comment on big plays, the star-studded halftime show and of course the commercials - multi million dollar productions by major brands that are often a draw in their own right.

The Super Bowl is the biggest stage for a broader battle that has intensified over the past year between the two behemoths of social media. By dominating online chatter during events such as the American football game, each company hopes to attract users and advertisers and capture a slice of the $70 billion spent annually on U.S. television advertising.

Both companies can serve up new ads in real time and can target specific audiences.

For years, the "second-screen" leader has been Twitter, which positioned its chatty, 140-character message platform as the go-to place for viewers to discuss events as they happen, from presidential debates to the Academy Awards. But Facebook, the world's largest social network with 1.2 billion users - roughly five times the size of Twitter - is now making aggressive moves to get into the real-time conversation itself.

"Twitter is better at engaging in the real-time conversation," said Quinn Kilbury, the brand director for Newcastle Brown Ale. "But Facebook is catching up quickly - and you can't match their scale."

This Super Bowl will be a major test for Facebook, which has spent the past year rolling out new features designed to close the gap with Twitter. The Menlo Park, California-based company has hired a team, including former TV producers, to help broadcasters extract insights about the Facebook users watching their programming, such as how a team's fans breakdown by gender or geographic region.

For the Super Bowl, Facebook will team directly with Fox Sports, the network owned by Twenty-First Century Fox Inc , which is airing the game, to showcase pre-game chatter from Facebook and Instagram users alongside typical game stats. A specially created website will chart user data and comments in real time. The company has also actively encouraged athletes such as Carolina Panthers running back DeAngelo Williams to